GDF15 and mitochondrial disease: Growth differentiation factor 15 (GDF15) and fibroblast growth factor 21 (FGF21) are representative metabokines that are responsive to mitochondrial diseases (Kalko et al., 2014; Lehtonen et al., 2016; Lovadi et al., 2017; Montero et al., 2016; Morovat et al., 2017; Yatsuga et al., 2015), and their expression is stimulated through ATF4, CHOP, and XBP1, key components of the integrated stress response (Chung et al., 2017b; Forsstrom et al., 2019; Jiang et al., 2014; Khan et al., 2017; Kim et al., 2013; Patel et al., 2019; Zhang et al., 2018).